1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use as a heat fixing apparatus to be mounted in a copying apparatus or a printer utilizing an electrophotographic recording technology or an electrostatic recording technology and a heater to be used in such apparatus, and more particularly to an image heating apparatus for heating an image by passing a recording material, bearing an image, through a nip portion between a heater and a backup member and a heater to be used in such apparatus.
2. Related Background Art
In the following, there will be explained an example of a prior image heating apparatus equipped in an image forming apparatus such as a copying apparatus or a printer, as an image heating apparatus (fixing apparatus) for heat fixing a toner image to a recording material.
In such image forming apparatus, an image heating apparatus of a heat roller type is widely employed as a fixing apparatus for heat fixing an unfixed image (toner image) of image information, formed and borne on a recording material (transfer sheet, electrofax sheet, electrostatic recording sheet, OHP sheet, printing paper, formatted paper etc.) by a transfer process or a direct process in an image forming process means utilizing a suitable image forming process such as an electrophotographic process, an electrostatic recording process or a magnetic recording process, as a permanently fixed image onto the surface of such recording material.
Recently an image heating apparatus of a film heating type is commercialized as a configuration capable of reducing a wait time from the entry of a print instruction to the start of a printing operation (quick start) and reducing the electric power consumption (energy saving). The image heating apparatus of such film heating type is proposed for example in Japanese Patent Application Laid-open Nos. S63-313182, H2-157878, H4-44075 and H4-204980.
The image heating apparatus of such film heating type is provided, as shown in FIG. 6, with a heater 13, a holder 11 for supporting the heater 13, a film (rotary member) 12 rotating in contact with the heater 13, and a pressure roller 18 which forms a nip portion with the heater 13 across the film 12. The pressure roller 18 is provided, on a metal core 19, with an elastic layer 19 formed for example of silicone rubber. The heater 13 is formed by printing, on a heat resistant substrate 14 for example of a ceramic material, a heat generating member 15 (also called resistor pattern), and a glass coating layer 16 for covering the heat generating member 15. For detecting the temperature of the substrate 14, a temperature detecting element 17 is provided. At the heating fixing of the toner image on the recording sheet, a current supply to the heat generating member 15 is controlled by unillustrated control means in such a manner that a temperature detected by the temperature detecting element 17 is maintained at a predetermined fixing temperature.
Also the arrangement of the heat generating member 15 is shown in a plan view in FIG. 7. In an example illustrated in (a) of FIG. 7, the heat generating member 15 is provided in one turn on the heat substrate 14. 210a indicates electrodes for connection with a connector in a main body of the printer, and 210b is a low-resistance conductor for connecting two heat generating members. The heat generating member 15 is proposed in various forms, and may be composed, as shown in (b) of FIG. 7, of a heat generating member 15 in a forward part and a low-resistance conductor (part of electrode) 210b in a return part. The recording sheet bearing the toner image is conveyed under pinching in the nip portion thereby being heat fixed.
The image heating apparatus applied as a fixing apparatus as explained above is also usable as an apparatus for improving a surface property such as glossiness by heating an image-bearing recording material, or a temporary fixing apparatus.
The image heating apparatus of such film heating type can be constructed as an apparatus of on-demand type utilizing members of a low heat capacity as a ceramic heater and a fixing film, and can be brought to a state heated to a predetermined fixing temperature by energizing the ceramic heater constituting a heat source only during execution of an image formation in the image forming apparatus, thereby providing advantages of significantly reducing a waiting time from the start of power supply in the image forming apparatus to a state capable of image formation (quick start property) and of significantly reducing the electric power consumption in a stand-by state (power saving).
However, in case of a continuous printing operation on small-sized sheets, there results a phenomenon of gradual temperature increase in an area not passed by the paper in the longitudinal direction of the fixing nip portion (temperature increase in sheet non-passing area). An excessively high temperature in the sheet non-passing area causes damages in various parts in the apparatus, and a printing operation on a large-sized sheet in a state with the temperature increase in the sheet non-passing area results in a high-temperature offset phenomenon in an area corresponding to the sheet non-passing area for the small-sized sheet.
As a countermeasure for such excessive temperature increase in the sheet non-passing area, it is conceived to provide the heater substrate with plural heat generating members corresponding to the sizes of the recording sheets used on the printer, but such method of forming plural heat generating members corresponding to the number of sizes is impractical as the recording sheets used on the printer have very many sizes.
Also there can be conceived a method, in a continuous printing operation on small-sized sheets, of increasing a gap between a preceding sheet and a succeeding sheet thereby relaxing the excessive temperature increase in the sheet non-passing area, but such method is associated with a drawback of significantly decreasing the number of the output sheets per unit time.
In order to suppress the excessive temperature increase in the sheet non-passing area without a significant decrease in the number of the output sheets per unit time, there is proposed, as disclosed for example in Japanese Patent Application Laid-open Nos. H5-19652 and H7-160131, a configuration of providing two electrodes along the longitudinal direction of the heater substrate and forming a heat generating member having a positive temperature coefficient (PTC) between such electrodes. An example of such configuration is shown in FIG. 8, in which there are shown a heater substrate 14 and electrodes 21, 22, and power supply connectors are connected to areas 21a, 22a. The two electrodes 21, 22 are provided along the longitudinal direction of the substrate 14, and a heat generating resistor 15 is connected between the two electrodes. FIG. 9 is an electrical equivalent circuit diagram of the heater shown in FIG. 8. As will be apparent from FIG. 9, this heater can be regarded as a configuration having numberless resistors 15r connected in parallel between the two electrodes 21 and 22 (hereinafter a heater of this type will be called a sheet-passing-direction current-feed type).
When a small-sized sheet is passed, an area E passed by the recording sheet shows a scarce temperature increase because the heat is taken away by the recording sheet. Therefore the heat generating member 15 in the sheet passing area does not show an increase of the resistance value thereby maintaining the current supply in the sheet passing area. On the other hand, in a sheet non-passing area, the heat generating member 15 shows an increase in the resistance value because of a temperature increase, thereby suppressing the current and suppressing the excessive temperature increase in the sheet non-passing area.
It is found, however, that such heater, when actually mounted in the fixing device, causes an unevenness in the distribution of heat generation in the longitudinal direction of the heater even when sheets are not passed. Such phenomenon is identified to result from resistances of the electrodes 21, 22. The two electrodes, provided along the longitudinal direction of the heater substrate 14, have a high conductivity but the resistance values thereof are not zero. Therefore the electrodes 21, 22 cause a voltage drop by the resistances thereof, whereby, even in the absence of the passing sheet, a side closer to the areas 21a, 22a in contact with the current supply connectors (left-hand side portion within the heat generating member 15 in FIG. 8) shows a larger heat generation and a side farther from the areas 21a, 22a (right-hand side portion within the heat generating member 15 in FIG. 8) shows a smaller heat generation.